A microwave filter is an electromagnetic circuit that can be tuned to pass energy at a specified resonant frequency. Accordingly, microwave filters are commonly used in telecommunication applications to transmit energy in a desired band of frequencies (i.e. the passband) and reject energy at unwanted frequencies (i.e. the stopband) that are outside the desired band. In addition, the microwave filter should preferably meet some performance criteria for properties, which typically include insertion loss (i.e. the minimum loss in the passband), loss variation (i.e. the flatness of the insertion loss in the passband), rejection or isolation (the attenuation in the stopband), group delay (i.e. related to the phase characteristics of the filter) and return loss.
A TE011 cavity filter assembly operating in single mode is commonly used in low-loss filters. It has a high, unloaded quality factor that makes it very attractive for a wide range of applications, including high-power applications.
A filter assembly may be made up of one or more resonators. Each resonator may consist of a cavity, which has interior surfaces that reflect a wave of a specific frequency. As more wave energy enters the cavity, it combines with and reinforces the standing wave, increasing its intensity. Although resonators are designed to generate waves of specific standing wave patterns or resonant modes, alternative resonant modes may also be formed. These unwanted modes may be degenerate and cause unwanted degradation to the filter performance.
Cavity shaping is well known in the art to separate the degenerate modes from a resonator cavity operating in the desired TE011 mode. However, such shaping increases the footprint of the TE011 cavity filter assembly and increases manufacturing complexity. Similarly, certain coupling techniques require the resonators to be stacked, with two resonators connected end on end and offset from one another.